Figuring out the current limiting resistor value:
Pick out the LEDs you want to use, then measure
the Lamp source voltage on the amp. The LED package
or spec sheet will tell you the diode (LED) Forward Voltage
and Forward Current in mA.
Plug those values into this handy calculator
and it will give you the needed value limiting resistor,
and size (1/4 watt, 1/2 watt,etc.) required:http://led.linear1.org/1led.wiz
There are also links at that site for series and parallel
calculators for you to figure out limiting resistors.
Remember that unlike incandescent lamps, LED's will
have most of the light projected in a much narrower range of view.
I have read (but not tried it) that lightly scuffing an LED
like that will cause a more even projection of light.
They're cheap, experiment with that.
I have replaced the lamps on my TS-940 with
high brightness LEDs, but managed to salvage the
rubber "booties" that covered the original meter lamps.
I covered the LEDs with these and it distributes the light well.
Good luck with your project and have fun!
73, Ken AD6KA